A variety of implantable medical devices (IMDs) are available to monitor physiological conditions of a patient, deliver therapy to a patient, or both. Typically, an IMD is coupled to one or more implantable leads that include electrodes. An IMD senses and monitors physiological signals or delivers electrical stimulation through the electrodes. For example, cardiac pacemakers are coupled to one or more implantable leads that include sensing and stimulation electrodes to sense cardiac electrical activity and deliver cardiac pacing pulses.
In some cases, an IMD is additionally coupled to and communicates with one or more physiological sensors. For example, some cardiac pacemakers receive signals output by oxygen saturation sensors, oxygen or carbon dioxide partial pressure sensors, intracardiac or intravascular fluid pressure sensors, temperature sensors, or accelerometers. Cardiac pacemakers typically use the signals output by physiological sensors to control or adjust the pacing therapy delivered to a patient.
Typically, sensors are located some distance away from the IMD, such as within the heart or vascular system of a patient, and communicate with the IMD over a two-conductor bus. The use of separate physiological sensing leads, i.e., implantable leads in addition to the one or more implantable leads that carry sensing or stimulating electrodes, increases the volume of lead material implanted within a patient. Increased volume of lead material adds to the complexity of the implant procedure.
In order to avoid the use of separate physiological sensing leads, some implantable leads incorporate sensing or stimulating electrodes, as well as physiological sensors. Such leads typically include additional conductors, i.e., separate from the conductors that are coupled to the electrodes, to couple the physiological sensor to an IMD. The additional conductors are used to isolate the physiological sensor from the electrodes and the stimulation and sensing circuitry of the IMD. Isolation serves to avoid unintended stimulation of body tissue via the physiological sensor, and reduces interference with the electrical sensing functions of the IMD. However, the presence of additional conductors increases lead diameter and stiffness, which can in turn increase the difficulty of the implantation procedure.